Liquid crystal displays (hereinafter, LCD) are flat panel display devices utilizing liquid crystal technology and semiconductor technology. LCD technology is based on the optical characteristics of the liquid crystal whose molecular arrangement is changed by an electric field.
A thin film transistor-LCD (hereinafter TFT-LCD) can consume low power, use low driving voltage, and be thin and light. The TFTs may be divided into polysilicon TFTs and amorphous silicon TFTs, based upon the material of the semiconductor layer used as a channel.
Processes for manufacturing the polysilicon TFT-LCD may be divided into low temperature processing (for example performed at a temperature of not more than 400.degree. C.) and high temperature processing (for example performed at a temperature of more than 900.degree. C.). In particular, low temperature processing performed at a temperature of not more than 350.degree. may be most appropriate for manufacturing an LCD having a large area since the processing can be performed on a glass substrate. This may make the polysilicon TFT-LCD more competitive than the amorphous silicon TFT-LCD.
However, trapping centers which may hinder the movement of electrons or holes may exist in both the low temperature processing and the high temperature processing due to grain boundaries in the polysilicon. The trapping centers may lower the mobility of the carriers and may increase the threshold voltage of the TFT.
Various techniques are generally used in order to manufacture a high performance TFT using low temperature processing.
First, a technique for forming a polysilicon film of good quality on a substrate without thermal transformation of the substrate is often used. Rapid crystallization by laser induction can be used to form a polysilicon film of good quality. Since crystallization of the silicon thin film using a laser can be performed without heating the substrate at a high temperature, the substrate may be damaged less. Thus, the mobility of the carriers in polysilicon which is formed by laser anneal may be larger than about 100 cm.sup.2 /Vs.
Second, a technique for forming a surface of good quality between the polysilicon film and an oxide film at a low temperature is used. When the trap density is high on the surface between the polysilicon film and the oxide film, the threshold voltage of the TFT may become higher. Although a plasma enhanced CVD (hereinafter, PECVD) may be appropriate for forming the oxide film at a low temperature, for example, 200.degree. C. to 400.degree. C., the PECVD may severely deteriorate the surface between the polysilicon film and the oxide film. There are various methods for reducing the deterioration of the surface, such as electron cyclotron resonance plasma CVD (ECR CVD) and radio frequency (RF) parallel-plate remote plasma CVD. In RF parallel-plate remote plasma CVD, it is possible to form a uniform thin film over a wide area. The polysilicon TFT having an oxide film formed by the remote plasma CVD may have a high mobility of about 400 cm.sup.2 /Vs and a low threshold voltage of about 1.5V.
Third, an annealing technique after metallization is often used. The characteristic of the TFT may be improved when the TFT is annealed at a temperature of 270.degree. C. in air. However, such an annealing method may impact the reproduction of the oxide film and the surface between the oxide film and the silicon film. In such an annealing method, it may be difficult to control humidity of air. Therefore, an annealing in which vapor is controlled at a low temperature is generally provided. The vapor may be most effective for annealing the TFT having an oxide film formed by the remote plasma CVD in the air at a low temperature.
The above-mentioned basic technologies are described in a publication entitled "High Quality SiO.sub.2 /Si Interfaces of Poly-Crystalline Silicon Thin Film Transistors by Annealing in Wet Atmosphere", Vol. 16, No. 5, May 1995, pp. 157-160 by Naoki Sano, et al.
In the above-mentioned conventional technology, the substrate may be annealed in a vapor atmosphere in order to remove trapping centers on the surface between the polysilicon film and the oxide film during the manufacture of the TFT. The oxide film is generally not formed by a general CVD method, but generally may be forced by the remote plasma CVD method in order to more effectively remove the trapping centers.